Wednesday, 19 November 2008

Writing to the MusicPal Display

Made a bit of progress with writing to the MusicPal display.
There is a device /dev/lcd which looked promising, but I didn't know what to do with it, but then I noticed that /dev/fb0 links to it. /dev/fb0 should be a linux framebuffer interface to the graphics hardware. Unfortunately most of the guides I could find on the internet were not about how to program the framebuffer, but how to set it up. I thought I was going to have to resort to reading /usr/include/linux/fb.h to try to understand it. I was pleased to find that someone (lesky?) has done that and published a bit of code to work with the framebuffer here.
He provided some example code to open the framebuffer device and extract information about it from the hardware as shown below:

#include
#include
#include
#include
#include

int main()
{
int fbfd = 0;
struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;
long int screensize = 0;
char *fbp = 0;
int x = 0, y = 0;
long int location = 0;

/* Open the file for reading and writing */
fbfd = open("/dev/fb0", O_RDWR);
if (!fbfd) {
printf("Error: cannot open framebuffer device.\n");
exit(1);
}
printf("The framebuffer device was opened successfully.\n");

/* Get fixed screen information */
if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
printf("Error reading fixed information.\n");
exit(2);
}

/* Get variable screen information */
if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
printf("Error reading variable information.\n");
exit(3);
}

/* Figure out the size of the screen in bytes */
screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

printf("xres=%d, yres=%d, bits_per_pixel=%d\n",
vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);

/* Map the device to memory */
fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
fbfd, 0);
if ((int)fbp == -1) {
printf("Error: failed to map framebuffer device to memory.\n");
exit(4);
}
printf("The framebuffer device was mapped to memory successfully.\n");

x = 100; y = 100; /* Where we are going to put the pixel */

/* Figure out where in memory to put the pixel */
location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
(y+vinfo.yoffset) * finfo.line_length;

*(fbp + location) = 100; /* Some blue */
*(fbp + location + 1) = 15; /* A little green */
*(fbp + location + 2) = 200; /* A lot of red */
*(fbp + location + 3) = 0; /* No transparency */

munmap(fbp, screensize);
close(fbfd);
return 0;
}

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